There is generally known a method of polymerizing a conjugated diolefin alone or an aromatic vinyl compound therewith in the presence of an anionic polymerization initiator, and coupling the resulting living polymer using a silicon halide compound such as tetrachlorosilane as a coupling agent to produce a branched conjugated diolefin (co)polymer rubber useful for automobile tires etc. However, such a coupling agent contains a halogen atom as a reactive site, and produces a halogen-containing compound such as LiCl as a by-product after the coupling reaction. On the other hand, it has been desired that a halide existing in a rubber is reduced.
For this purpose, various coupling agents of the non-halogen family have been proposed. For example, JP-A-7-2958 proposes a method for producing a conjugated diene-based polymer having a specific branched structure, using an alkoxysilane as the coupling agent.
However, when the alkoxysilane is used as the coupling agent, the resulting branched conjugated diolefin (co)polymer rubber has the problem not observed in the coupling reaction with a conventional polysilicon halide compound that a polymer component having a structure other than a desired branched structure is formed with the elapse of time because of hydrolysis and condensation of alkoxysilyl group.
In order to solve this problem, JP-A-10-25313 proposes a method of polymerizing a conjugated diene-based monomer using an organic lithium compound as an initiator in a hydrocarbon solvent to produce a living polymer, and coupling it using an alkoxysilane compound, followed by treatment by the addition of at least one kind of Lewis base selected from a tertiary diamine, a chain ether having two or more oxygen atoms in one molecule or a cyclic ether to the above-mentioned hydrocarbon solvent, as a method for producing a halogen-free branched conjugated diene-based polymer effectively and stably.
However, also in this method, there are variations in the Mooney viscosity of the resulting polymer, and it is difficult to restrain changes in the Mooney viscosity of the polymer with the elapse of time.
Patent Document: JP-A-7-2958
Patent Document: JP-A-10-25313